leetcode - Multiply Strings

Given two numbers represented as strings, return multiplication of the numbers as a string.

Note: The numbers can be arbitrarily large and are non-negative.

class Solution {
public:
std::string multiply(std::string num1, std::string num2) {
if(num1 == "0" || num2 == "0") return "0";
int l1 = num1.size(),l2 = num2.size();
std::vector<int> vec1(l1,0);
std::vector<int> vec2(l2,0);
std::vector<int> res(l1+l2,0);
for (int i = 0; i < l1; i++)
{
vec1[i] = num1[i] - '0';
}
for (int i = 0; i < l2; i++)
{
vec2[i] = num2[i] - '0';
}
for (int i = 0; i < l1; i++)
{
for (int j = 0; j < l2; j++)
{
res[i+j+1] += vec1[i] * vec2[j];
}
}
std::string s;
for(int i = l1 + l2 - 1; i >= 0; i--)
{
if(res[i] > 0) res[i-1] += res[i] / 10;
res[i] %= 10;
s = (char)(res[i] + '0') + s;
}
return s[0] == '0' ? s.substr(1) : s;
}
};